This application claims the priority of German Application 19754964.0, filed Dec. 11, 1997, the disclosures of which is expressly incorporated by reference herein.
The invention relates to a device for supplying electricity to a motor vehicle according to the preamble of claim 1.
A device of this type is known, for example, from German Patent Document DE 43 40 350 C2. This known device has a rechargeable battery, a capacitor which can be switched in parallel to the battery, and a circuit arrangement in the form of a logic circuit arranged between the battery and the capacitor. When the starter in the vehicle is actuated, the logic circuit defines a time window and carries out at least one voltage query. As a function of the result of this voltage query, the logic circuit switches the capacitor in parallel to the battery. The capacitor is preferably switched in parallel only when the voltage of the battery within the time window falls below a predetermined value. By means of this known circuit arrangement, with the capacitor switched in parallel to the battery, the starting of the vehicle is still ensured when the battery is almost discharged. However, with the parallel connection of the capacitor with the battery, the maximum capacitor voltage cannot exceed the value of the battery voltage. This known circuit arrangement is therefore not suitable for storing in the capacitor a high excess of energy for a short time and thus for effectively recharging a discharged battery.
In addition, a circuit arrangement for supplying electricity to a motor vehicle is known from German Patent Document DE 195 22 563 A1, in the case of which the energy stored in a capacitor, particularly the electric energy generated during a recuperative braking, is fed to a rechargeable battery in a controlled manner. However, this known circuit arrangement discloses no details concerning the type of the control or the ratio of the maximally possible capacitor voltage to the battery voltage.
Furthermore, with respect to the technical environment, reference is made to European Patent Document EP 0 568 655 B1, from which a device is known for supplying electricity to a motor vehicle which has two chargeable batteries of different nominal voltages and a circuit arrangement in the form of a voltage transformer arranged between the batteries.
It is an object of the invention to provide an improved device of type for supplying of electricity to a motor vehicle such that, on the one hand, an energy excess which is available for a short time is effectively utilized for charging a vehicle battery and simultaneously the service life of the rechargeable vehicle battery is increased.
The use of a capacitor (such as a Power Cap or Super Cap) whose nominal voltage and thus its maximally possible voltage is preferably several times higher than the nominal voltage of the battery is essential to the invention. By using such a buffer capacitor with a large voltage variation range as the energy accumulator, the voltage at the capacitor can be significantly increased beyond the battery voltage in order to be able to store in the best possible manner, a short-term energy excess, as, for example, resulting from recuperative braking. The battery is charged in a controlled manner by means of this capacitor by using a voltage transformer, preferably a DC/DC converter.
In a particularly advantageous further development of the invention, the charging of the battery is controlled by means of the capacitor by way of the voltage converter such that the charged capacitor is maximally discharged until a capacitor voltage is reached which is approximately equal to the momentary actual voltage of the battery. By means of this advantageous further development, a circuit arrangement can be used as a voltage transformer which only has to carry out a xe2x80x9cdownwardxe2x80x9d transformation in the sense of a voltage reduction starting from the capacitor voltage. As the result, the voltage transformer can be built, in a particularly simple manner, at reasonable cost, between the battery and the capacitor.
The invention also includes an expanded voltage transformer in such a manner that, in the reverse direction, the capacitor can be charged by way of the battery to a voltage whose value is larger than the value of the battery voltage.
The circuit arrangement according to the invention is used in the case of motor vehicles with a chargeable battery which has a higher than conventional nominal voltage (for example, 36 V instead of 12 V) in order to ensure the supply of high-power consuming devices whose number is constantly increasing in motor vehicles.
With a device for supplying electricity according to the invention, on the one hand, an energy excess which is available for a short time is effectively utilized and, on the other hand, a variable multivoltage electrical wiring is permitted.